Low brightness functional pigment from process by-product

ABSTRACT

A low brightness pigment and method is provided from a heretofore rejected waste kaolin stream. Kaolin processing rejects are high in iron-stained TiO 2 . After separating the stained TiO 2  from the high brightness kaolin and concentrating the resulting fraction, the material can be processed to produce a low brightness pigment suitable of many applications where lower brightness is desirable, such as coating paper board without mottling.

This application is a division of application Ser. No. 07/643,885 filedJan. 18, 1991, now U.S. Pat. No. 5,154,767.

TECHNICAL FIELD

The present invention relates to mineral pigments for use in the paperindustry and more particularly relates to a method of producing lowbrightness pigments from beneficiation process by-products.

BACKGROUND ART

Natural occurring clays vary considerably in their color properties,even when produced from mines in the same locality or even fromdifferent sites in the same mine. Natural occurring kaolin clay depositscontain discoloring contaminants, for example, iron and titaniumminerals. Titanium minerals in kaolin usually occur as discoloredparticles and these, coupled with iron oxides and other ferriferousmaterial, are largely responsible for the yellow-brown shade of manykaolins. Often a clay is rejected as being unsuitable for commercial usesolely on the basis of brightness even though its other physicalproperties, such as the viscosity of clay-water slurries and particlesize, are within desired limits.

Much attention and research in the clay industry has focused ondeveloping processes to increase the brightness of clays. In the waterwash process, crude clay is slurried, degritted, fractionated to thedesired particle size and the resulting fractions leached with zinc orsodium dithionite at a pH of 3.5-4.0 to improve both brightness andshade. Brightness improvements due to dithionite leaching can be quitesubstantial; however, the increase in brightness is generallyinsufficient to make high brightness products in the range of 90 TAPPIbrightness, or to make lower brightness products when highly discoloredcrude clays are employed. Therefore, other processing methods, such asselective sedimentation, magnetic separation, froth flotation, andselective flocculation, have been developed for use in conjunction withdithionite to improve the brightness of the leached products. Thesemethods can be employed to produce both standard and high brightnessproducts from highly discolored starting materials by removing much ofthe iron stained titanium and ferriferrous material prior to theleaching step. Selective sedimentation, magnetic separation, frothflotation and selective flocculation are processes designed to removethe highly discolored titanium impurities and iron oxides. Thedithionite leach step solublizes and removes a portion of the amorphousferriferrous impurities on the clay surface.

The most successful processes developed to date to improve thebrightness of clays are those in which impurities, such as titanium andiron compounds, are removed by selective sedimentation, selectiveflocculation, more commonly referred to as Differential Flocculation ofAnatase (DFA), and magnetic separation.

The selective sedimentation process is exemplified by U.S. Pat. No.3,371,988 to Maynard et al. The process of this patent has been found tobe disadvantageous in that relatively long sedimentation periods arerequired.

U.S. Pat. No. 3,857,781 to Maynard exemplifies the DFA process which isa substantial improvement over U.S. Pat. No. 3,371,988. This processinvolves the use of an anionic, high molecular weight polymer in thepresence of specific inorganic salts which flocculates and settles outmuch of the iron-stained titanium dioxide contaminates at extremelyrapid sedimentation rates.

A third process exemplified by U.S. Pat. No. 4,424,124 to Iannicelliinvolves the removal of iron oxides and iron stained TiO₂ impurities bymagnetic separation. All three processes, selective sedimentation, DFA,and magnetic separation produce a substantial amount of reject materialwhich have heretofore been discarded.

A fourth process, not yet widely used, is called froth flotation whichutilizes frothing agents to remove TiO₂ impurities. These froths rise tothe surface and are removed to leave an improved brightness product.

The present invention is directed to a process for producing a usablepigment from the reject material from the above processes or a similarprocess which produces a low brightness, titanium dioxide-rich reject.

SUMMARY OF THE INVENTION

In the present invention a method for producing a low brightness, highyellow index clay pigment from a clay containing iron-stained titaniumdioxide is described. The clay is dispersed in water to create a kaolindispersion which is separated into an aqueous high brightness fractionand an aqueous lower brightness fraction. This lower brightness fractioncontains substantially 4-12% by weight of dry clay TiO₂ andsubstantially between 0.5-2.5% by weight of dry clay Fe₂ O₃. The lowerbrightness fraction is concentrated to form a kaolin suspension whichcontains kaolin particles and iron-stained TiO₂ particles. The kaolinsuspension is washed to remove excess salts to form a kaolin pigmentslurry.

In another embodiment of the present invention, a low brightness claypigment is provided which comprises kaolin particles containingsubstantially between 5-12% by weight of dry clay TiO₂, substantiallybetween 0.5-0.25% by weight of dry clay Fe₂ O₃. This pigment has a TAPPIbrightness substantially between 60 and 80.

It is accordingly one object of the present invention to provide amethod for producing a TiO₂ -rich pigment from the rejects of a mineralbeneficiating process.

It is another object of the present invention to provide a method forproducing a mineral pigment from a mineral flocculated by means of ananionic polymer.

It is a further object of the present invention to provide for a mineralpigment which has been recovered from the rejects of a sedimentationprocess.

It is a further object of the present invention to provide for a mineralpigment which has been recovered from the rejects of a magneticseparation process.

It is a further object of the present invention to provide for a mineralpigment which has been recovered from the rejects of a froth flotationprocess.

It is a further object of the present invention to provide for a mineralpigment which has been produced from a mineral flocculated by means ofan anionic polymer.

It is yet another object of the present invention to provide a pigmentwhich has a low brightness high yellow index for board coatingapplications to reduce mottling yet provide superior hiding power.

It is yet another object of the present invention to provide a pigmentwhich may be utilized to adjust brightness and simultaneously improveprinting qualities.

It is a further object of the present invention to provide for a paperproduct which incorporates a mineral pigment which has been recoveredfrom the rejects of a beneficiating process.

It is a still further object of the present invention to provide for apaper product which incorporates a low brightness pigment.

According to the present invention, there is provided a method forproducing pigments from reject kaolin which has been separated from thehigh brightness product flow stream. The present invention also providesfor a novel pigment produced from the rejects of mineral beneficiatingprocesses.

By way of the present method, there is further provided pigments whichcan be utilized in the production of a variety of paper and paper boardproducts.

DETAILED DESCRIPTION

In the process of the present invention, useful pigments are producedfrom kaolin clay by first dispersing the clay in a convenient mannerwell known in the art and then separating the dispersion into a highbrightness, low yellow index fraction and a low brightness high yellowindex fraction. This separation may be done by the separation methodsdiscussed earlier, such as selective sedimentation, selectiveflocculation (DFA), magnetic separation or froth flotation, either aloneor in combination. Heretofore, only the higher brightness, lower yellowmaterial was subjected to further processing to a final product and thelower brightness fractions were either reprocessed for furtherextraction of high brightness material or, if no more high brightnessmaterial could be extracted, discarded. It is a discovery of the presentinvention that these "rejects" can be made into a useful pigment.

After the dispersed clay has been separated into a high brightness (+85TAPPI), low yellow index fraction, and a low brightness (<80 TAPPI) highyellow index fraction, the clay particles and iron-stained TiO₂ in thelow brightness fraction are concentrated to produce a mineralsuspension. This concentration step permits the efficient handling andfurther processing of the pigment.

Concentration may be achieved by filtration, frothing or sedimentation,but flocculation followed by hydro-separation with filtration ispresently preferred. This produces a flocculent typically having asolids content of about 65 percent. If the DFA process is employed, itmay be seen that separation of fractions and concentration of pigment isachieved simultaneously. If separation is achieved by magneticseparation, the "rejects" are already dispersed at low solids (1-3%) inthe magnet flush water. If concentration is achieved by filtration,later dispersion may not be necessary. However, it is presently believedthat flocculation of the pigment from the flush water would still bepreferred. If separation is by selective sedimentation, the rejectslurry may already be in a dispersed state at high enough solids forfurther processing. If not, it too may be flocculated or filtered. Ifthe separation is achieved by froth flotation, the froth issubstantially concentrated also, although further concentration may bedesired.

After the TiO₂ -rich pigment is concentrated, the pigment is thoroughlywashed to remove excess salts accumulated in earlier processing. Thiswashing may be achieved by elutriation of the flocculent at low solidsand then filtered, centrifuged or by other means known to the art, butit is most preferred to disperse the concentrate (if a polymerfloccule), dilute the slurry, screen, re-flocculate and filter as morefully described later. The pigment can be further washed on the filtersby using sprays or multiple filtration. It should be noted that it isgood practice during the process to periodically screen or "degrit" theconcentrate slurry while it is in the dispersed, and often more dilute,state.

After washing, the filter cake is dewatered. This may be achieved byredispersing the pigment, typically with an organic dispersant, andspray drying or drying the filter cake directly and milling.Alternatively, a portion of the dispersed pigment may be spray dried andblended back into the dispersion slurry to increase the solids contentsufficiently for use as a slurried pigment.

By way of example, a more preferred process embodiment utilizing therejects from the DFA process will be discussed in detail; however, it isunderstood that conventional modification of the method to utilize therejects from magnetic separation, selective sedimentation, frothflotation or other processes would be within the scope of the presentinvention and that the rejects, separated by whatever method, aresubstantially similar.

This more preferred embodiment for preparing useful low brightness, highyellow index pigments from kaolin involves dispersing a crude kaolinclay in water using excess dispersants and degritting as required.Following high shear mixing, additional salts, preferably sodiumchloride, are added with dilution and the dispersion is aged. Thedispersion is further diluted and flocculated with an anionic, highmolecular weight polymer to separate the kaolin dispersion into a highbrightness and low brightness fraction. The low brightness floccule,rich in iron-stained TiO₂, is concentrated and removed byhydro-separation. For a more detailed discussion of the DFA separationprocess, see U.S. Pat. No. 3,857,781 to Maynard which is herebyincorporated by reference.

The floccule so produced, and heretofore termed "rejects", isredispersed at high shear agitation, adding a suitable dispersant,typically sodium hexametaphosphate. The solids content during thisagitation preferably should be adjusted to about 30-50 percent byweight, typically about 40%.

Once the flocculated material has been redispersed, it is preferred thatthe solids content thereof is adjusted as necessary in order that thedispersion may be degritted by passing it through a screening process.At this point the solids content should be adjusted to about 25 to 50wt. %, typically about 35%.

Following degritting, if elected, the mineral slurry is reflocculatedand filtered to remove excess salts. Reflocculation is achieved byadjusting the solids content to between about 20 to about 30 wt. % andadjusting the pH of the slurry to between about 3.0 and 5.5, with pH 4being preferred. In a preferred embodiment the pH was initially adjustedto below about 5.5 by the addition of a strong acid, e.g. sulfuric acid.The final pH adjustment was achieved by the addition of alum. Afterfiltering, the filter cake is then either redispersed and spray dried oracid dried and physically sized, e.g. by air classification, accordingto the desired final product characteristics. The resulting pigmentscontain 5-12% by weight TiO₂ and 0.5-2.5% Fe₂ O₃. More typically, thepigment contains 7-10% TiO₂ and 1-2% Fe₂ O₃. Throughout, all Ti and Fecompounds present are reported as if 100% in the form of its respectiveoxide.

An alternative, through less preferred technique for removing excesssalts from the TiO₂ rich floc involves elutriating or washing the flocswith water under low agitation conditions and then filtering and dryingas described above. Many other techniques are well known in the industryto alternately disperse, flocculate and re-disperse to beneficiatekaolin slurries and these same techniques can be employed in practicingthe present invention.

As discussed above, the present process may be utilized to recoverpigments from the rejects of a beneficiation process that utilizespolymers as flocculating agents. Alternatively, the initial startingmaterial may be a mineral, such as a crude clay, which includes titaniumand/or iron compound impurities. In such a case, in order to obtainpigments according to the present invention the titanium and/or ironcompounds must first be concentrated.

In order to concentrate the iron-stained titanium impurities the claymust be subjected to a beneficiation process which causes the titaniumimpurities to be removed and concentrated. The preferred procedure forthis concentration of titanium impurities follows the general procedureset forth in U.S. Pat. No. 3,857,781. That is, the mineral, e.g. clay,is dispersed, treated with a salt and aged. After aging with the salt,the dispersion is treated with a polymer which effects sedimentation ofthe titanium impurities by forming titanium polymer flocs whichsubsequently settle out.

Preferred salts used for selective flocculation of the titaniumimpurities include sodium chloride, sodium sulfate, sodium carbonate,potassium chloride, sodium nitrate and ammonium chloride, with sodiumchloride being more preferred. The amount of salt utilized toeffectively treat clay mineral dispersions has been found to be betweenabout 4-50 pounds per ton of dry clay, and more preferably about 12pounds per ton of dry clay. The salt should be added to a 25-45 wt. %solids mineral dispersion, and more preferably to about a 40 wt. %solids mineral dispersion. The initial mineral dispersion may beproduced by utilizing a suitable dispersion agent such as sodiumhexametaphosphate and/or sodium metasilicate.

The salt treated clay should be aged for a period of from 5 to 24 hoursunder static conditions. Otherwise the salt treated dispersion may beaged for 1 to 5 hours under mild agitation. The latter aging process ismore suitable for commercial production.

After aging, the dispersion should be adjusted to have a solids contentbetween about 10 to 30 wt. %, and preferably 20 wt. %. During orimmediately following dilution the polymer flocculation agent is addedat about 0.01 to 0.3 pounds per ton of dry clay to form iron-stainedtitanium-polymer flocs which settle out, thus forming the titania-richclay concentrate utilized to produce the pigments according to thepresent invention.

Preferred polymers utilized as flocculation agents include Nalco 635 and675, a water soluble, strongly anionic, polyacrylamide polymer with amolecular weight in excess of one million, produced by Nalco ChemicalCompany, Chicago, Ill. and Betz 1200, 1210, 1220 and 1230, a watersoluble, strongly anionic, organic copolymer of acrylamide having amolecular weight in excess of one million, available from BetzLaboratories, Inc., Trevose, Pa. Other suitable polymers employedcomprise water soluble, strongly anionic products produced by thepolymerization of acrylamides or copolymers of acrylamide and othermonomers polymerizable therewith such as acrylic acid, methacrylic acid,etc.

It is important to distinguish the above titanium concentration processfrom the beneficiation process of U.S. Pat. No. 3,857,781. In thisregard there are a number of considerations that effect the propertiesof the final product of U.S. Pat. No. 3,857,781 as discussed therein.U.S. Pat. No. 3,857,781 is primarily interested with the beneficiatedclay, and according selects process parameters to insure desiredproperties of the beneficiated clay.

In contrast to U.S. Pat. No. 3,857,781, the present process is utilizedto obtain a concentrate of titanium impurities for subsequent productionof a pigment having a high concentration of titanium compounds. Theproperties of the "beneficiated" mineral or clay in the above processmay be of little concern, since it may be subjected to other treatmentprocesses. Accordingly, the process parameters may be selected tooptimize or control the amount of titanium concentrated.

The following example is illustrative of a preferred embodiment used forproducing a clay pigment according to the present invention which is notto be considered as limited thereto. In this example and throughoutpercentages are by weight unless otherwise indicated.

EXAMPLE

A 55-gallon drum of polymer flocculated anatase reject material from aDFA hydroseparator underflow stream was used. The contents of the drumwere agitated with a Lightnin-type mixer equipped with a marine-styleimpeller. The agitated contents were sampled and found to have 47.25 wt.% solids. The analytical technique utilized for determining the percentsolids by specific gravity had to be modified to account for the densitychanges caused by flocculating the slurry with polymers. For thispurpose, based on the previous processing of the clay, a correctionfactor of +0.3 g/100 cc has used. Thus the 653 pounds of slurrycontained 308 pounds of bone dry, iron-stained TiO₂ rich clay. Theslurry pH was measured at 10.4.

The contents of the drum were transferred to a 30-inch diameter tankplaced under a 25 H.P. Cowles dissolver installed with an 18-inchdiameter dissolver blade. In order to break up the flocs, and todisperse the clay, one pound per ton B.D. clay (lbs/TC) of sodiumhexametaphosphate (Calgon) was added and the slurry was agitated for 5minutes with the Cowles mixer. The temperature of the slurry wasobserved to rise from 24° to 27° C. The Cowles dissolver was turning at750 RPM, yielding a tip speed (on the 18-inch impeller blade) of 3534.3feet per minute.

The slurry was next transferred from the tank under the mixer through ascreener and into a storage tank. In order to pass the slurry throughthe screener, it had to be diluted to about 40 wt. % solids. Thenecessary dilution was conducted by adding water to the slurry in thetank under the mixer. The slurry was pumped directly to a 48-inchdiameter Midwestern vibrating screener, equipped with a 325 mesh screen.The material that passed through the screen was caught in a catch-bucketon the inlet to a centrifugal pump, which moved the material to apotbellied tank. A total of 77.7 gal. slurry was recovered. Analysisrevealed a solids content of 36.27 wt. %, for a total of 302.8 pounds ofB.D. clay, and a yield of 98.31%. The purpose of the screening was todegrit the slurry. About one liter of grit was found to have remained onthe screen at the end of the transfer.

The exact solids content of the dispersed slurry was then used to dilutethe slurry to a solids range of 22.5 to 23.5 wt. %. A solids content of23.33 wt. % was actually achieved. This slurry was then flocculated byadding sulfuric acid (H₂ SO₄) until a pH of 5.03 was achieved, and thenadding alum to a pH of 3.9. This resulted in the addition of 7.5 lbsacid/ton B.D. clay, and 6.0 lbs alum/ton clay (dry basis). A smallportion of this slurry was filtered on a buchner funnel filter toprovide the "heel" required for the full discharge on the pilot rotaryvacuum filter, a 1-foot diameter by 1-foot wide pilot rotary vacuumfilter. Although it took a significant amount of time to accomplish thefiltration, no problems were encountered. The filtration parameters aregiven in Table I below.

                  TABLE I                                                         ______________________________________                                        Filtration area  3.14 ft.sup.2                                                Drum speed       1.7 RPM                                                      Cloth, porosity  4-6 cfm                                                      Vacuum           22-24 in Hg                                                  Wet filter cake  96 lbs/m                                                     Cake solids      60.13%                                                       Dry clay         57.7 lbs/hr                                                  Filtration rate  18.4 lbs dry clay/hr/ft.sup.2                                Filtrate solids  0.38%                                                        ______________________________________                                    

A total of 477 lbs. of filter cake was obtained. Moisture analysisrevealed a solids content of 60.33%, for a total of 288 lbs. B.D. clay,and a yield of 95.1%.

The filter cake was placed in a small tank, under a small CowlesDissolver and dispersed in two batches. To each batch was added9.2lbs./TC fresh as-received Colloid 211, and 4.1 lbs./TC soda ash todisperse the filtered cake. The slurry was 6.9 (target range was6.8-7.2). In addition, sufficient water was added to reduce the solidscontent down to 52 wt. %, to facilitate screening. Tip speed on thesmall Cowles dissolver was maintained at about 3500 RPM. The slurry wasscreened again, through 325 mesh screen, and then spray dried.

A large spray dryer was used to dry the entire lot of material, as perstandard procedure. Outlet temperature was held at 220° F. by varyingthe feed rate, and the inlet temperature was maintained at 1000° F. byvarying the gas to the air heater. Product data is given in Table IIbelow.

                  TABLE II                                                        ______________________________________                                        Spray dried moisture                                                                             0.6%                                                       Brightness (TAPPI T-534)                                                                         71.6                                                       Whiteness          32.8                                                       Yellowness         15.2                                                       +325 mesh grit     0.002%                                                     Chemical Analysis                                                             TiO.sub.2          8.1%                                                       Fe.sub.2 O.sub.3   1.5%                                                       Al.sub.2 O.sub.3   36.5%                                                      SiO.sub.2          43.7%                                                      Particle Size (Sedigraph)                                                     +10μ            4.5%                                                       +5μ             16.0%                                                      -2μ             64.5%                                                      -1μ             55.0%                                                      -0.5μ           46.0%                                                      -0.2μ           25.5%                                                      ______________________________________                                    

The clay made by the above-described process may be utilized as acoating in applications wherein high brightness is not required. Forexample, it has been discovered that clay processed by the presentinvention is usable as a precoat for a paper or paperboard with lowbrightness base stock. The use of the low (60-80 TAPPI) brightness clayreduces the contrast between the coating and the base stock, therebyreducing the mottled appearance of the finished sheet. Furthermore, theiron-stained titanium dioxide present in the clay pigment due to theconcentration effect of prior processing (5-12% by weight) gives hidingpower superior to that attainable with clay alone.

Typically, if titanium dioxide were added to a standard coating clay,the brightness of the coating would increase and the mottling would beexaggerated due to the uneven coating distribution over the rough,unbleached fiber surface. To lessen this potential problem, a dye orcolorant would have to be added to the coating. This procedure reducesthe contrast between the coating and the base stock. The product of thepresent invention would be used without additional titanium dioxide andwithout dyes or colorants.

The product of the present invention has also been discovered to beparticularly useful in woodfree paper grades. The use of greater amountsof calcium carbonate in coatings today for woodfree premium paper gradesmay result in the finished sheet exceeding the brightness limit for aspecific grade. In mills were several grades of paper are produced, andvarious brightness levels are required, base sheet brightness cannot bevaried in the pulp mill and be cost-effective. The coating formulation,therefore, must be the agent of change. Dyes and colorants can controlthe brightness and shade but the pigment of the present invention willnot only control shade and color, it will act as a kaolin clay pigment,providing improved coated sheet properties such as paper gloss, andimproved print quality, as clay yields better printed gloss than calciumcarbonate.

Although the invention has been described with reference to particularmeans, materials and embodiments, from the foregoing description, oneskilled in the art can ascertain the essential characteristics of thepresent invention and various changes and modification may be made toadapt the various uses and characteristics thereof without departingfrom the scope of the present invention as described in the claims thatfollow.

We claim:
 1. A method for producing a low brightness, high yellow indexkaolin pigment slurry from a kaolin clay containing iron-stained TiO₂,the method comprising:dispersing the kaolin clay in water to form akaolin dispersion; separating the kaolin dispersion into an aqueous highbrightness fraction and an aqueous lower brightness fraction, the lowerbrightness fraction having a TiO₂ content substantially between 5-12 %by weight dry clay and an Fe₂ O₃ content substantially between 0.5-2.5%by weight dry clay; concentrating the lower brightness fraction to forma kaolin suspension containing kaolin particles and iron-stained TiO₂particles; wherein the step of concentrating includes flocculating atleast a portion of the iron-stained TiO₂ particles and the kaolinparticles from the lower brightness fraction with a flocculating agent;and recovering the flocculent containing the TiO₂ and kaolin particlesby hydroseparation to form a recovered kaolin suspension; and washingthe recovered kaolin suspension to remove excess salts to form saidkaolin pigment slurry; and wherein the washing step includes, dispersingthe recovered kaolin suspension with a dispersion agent; diluting thedispersed kaolin suspension; flocculating the diluted suspension; andfiltering the flocculated suspension to form a filter cake therebyremoving excess salts and dispersing the filter cake with a dispersingagent to form the kaolin pigment slurry.
 2. A method for producing a lowbrightness, high yellow index kaolin pigment from a kaolin claycontaining small quantities of iron-stained TiO₂, the methodcomprisinga) dispersing the kaolin clay in water to form a kaolindispersion; b) passing the kaolin dispersion through a screen to removegrit; c) selectively flocculating the iron-stained TiO₂ by adding astrongly anionic, high molecular weight polymer floccing agent; d)separating the TiO₂ polymer flocculent from the kaolin dispersion byhydroseparation and recovering the flocculent from the kaolin dispersionto produce a high solids floc; e) dispersing the high solids floc athigh shear in the presence of a first dispersing agent; f) passing thedispersed floc of step (e) through a screen to remove grit; g)reflocculating the degritted dispersed floc to produce a reflocculate;h) filtering the reflocculate to produce a filter cake; and i)redispersing the filter cake in the presence of a second dispersingagent to produce a kaolin pigment slurry having a TiO₂ content of 5-12%by weight on a dry clay basis and a Fe₂ O₃ content of 0.5 to 2.5% byweight on a dry clay basis.
 3. The method of claim 2 wherein firstdispersing agent and the second dispersing agent are non-identical andthe second dispersing agent is an organic dispersing agent.
 4. Themethod of claim 2 further including the step of spray drying at least aportion of the pigment slurry.